The fluorescent lamp is used as a popular lighting apparatus. However, growing concern for environmental pollution increases the demand for light sources which are free of mercury, and light emitting devices based on light emitting elements such as light emitting diodes (hereinafter referred to as LEDs) or laser diodes (hereinafter referred to as LDs) are attracting much attention.
Light emitting devices based on the light emitting elements are small in size, high in power efficiency and are capable of emitting light of vivid colors. Furthermore, such light emitting elements are semiconductor devices free of failure such as burnout. These light emitting elements also have such advantages as excellent startup characteristics and high tolerance to vibration and to repetitive turning ON/OFF. Because of such excellent characteristics, light emitting devices based on light emitting elements such as LEDs and LDs are used widely in various light sources.
Regarding white light emitting devices based on LEDs, known methods consist of combinations of the following two types.
The first type is a light emitting device that combines a blue LED and a YAG fluorescent material which emits yellow light. The light emitting device emits white light by blending blue light and yellow light, whereas the YAG fluorescent material is excited by the blue light emitted by the blue LED. The light emitting device has the advantages of reduction in power consumption, easy control of the LED operation and good color blending performance, and therefore has been widely used.
The second type is a light emitting device that combines a blue LED, a green LED and a red LED. This light emitting device is the so-called three-wavelength light emitting device that generates white light by blending the light from the three LEDs. The light emitting device is capable of reducing the power consumption, and has a wide range of display colors for light that passes through the liquid crystal. There is also a light emitting device that combines a blue LED, a blue-green LED, an orange LED and a red LED in order to improve the efficiency and color rendering performance (for example, Patent Document 1).    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2003-45206